1. Field of the Invention
Embodiments disclosed herein relate generally to methods and apparatus for drilling and completing well bores. More specifically, embodiments disclosed herein relate to apparatus for a frac plug and methods of isolating zones using a frac plug. More specifically still, embodiments disclosed herein relate to an isolation device for frac plugs.
2. Background Art
In drilling, completing, or reworking wells, it often becomes necessary to isolate particular zones within the well. In some applications, downhole tools, known as temporary or permanent bridge plugs, are inserted into the well to isolate zones. The purpose of the bridge plug is to isolate a portion of the well from another portion of the well. In some instances, a frac plug (or fracturing plug) is used to isolate perforations in the well in one section from perforations in another section of the well. In other situations, there may be a need to use a bridge plug to isolate the bottom of the well from the wellhead. These plugs may be removed by drilling through the plug.
Drillable plugs generally include a mandrel, a sealing element disposed around the mandrel, a plurality of backup rings disposed around the mandrel and adjacent the sealing element, an upper slip assembly and a lower slip assembly disposed around the mandrel, and an upper cone and a lower cone disposed around the mandrel adjacent the upper and lower slip assemblies, respectively. FIG. 1 shows a section view of a well 10 with a wellbore 12 having a plug 15 disposed within a wellbore casing 20. The plug 15 is typically attached to a setting tool and run into the hole on wire line or tubing (not shown), and then actuated with, for example, a hydraulic system. As illustrated in FIG. 1, the wellbore is sealed above and below the plug so that oil migrating into the wellbore through perforations 23 will be directed to the surface of the well.
The drillable plug may be set by wireline, coil tubing, or a conventional drill string. The plug may be placed in engagement with the lower end of a setting tool that includes a latch down mechanism and a ram. The plug is then lowered through the casing to the desired depth and oriented to the desired orientation. When setting the plug, a setting tool pulls upwardly on the mandrel, thereby pushing the upper and lower cones along the mandrel. This forces the upper and lower slip assemblies, backup rings, and the sealing element radially outward, thereby engaging the segmented slip assemblies with the inside wall of the casing.
As shown in FIGS. 1B and 1C, a frac plug 30 includes a mandrel 32 having an axial bore 34 therethrough and a seat 36 disposed within the bore 34. The seat 36 is configured to receive a ball 38 to isolate zones of a wellbore and allow production of fluids from zones below the frac plug 30. The ball 38 is seated in the seat 36 when a pressure differential is applied from across the seat 36 from above. For example, as fluids are pumped from the surface downhole into a formation to fracture the formation, thereby allowing enhanced flow of formation fluids into the wellbore, the ball 38 is seated in seat 36 to maintain the fluid, and therefore, fracturing of the formation in the zone above the plug 30. In other words, the seated ball 38 may prevent fluid from flowing into the zone isolated below the frac plug 30. The ball 38 may be dropped from the surface or may be disposed inside the mandrel 32 and run downhole within the frac plug 30.
At high temperatures and pressures, i.e., above approximately 300° F. and above approximately 10,000 psi, the commonly available materials for downhole balls are not reliable. Furthermore, a conventional ball seat 36 includes a tapered or funnel seating surface 40. The ball 38 makes contact with the seating surface 40 and forms an initial seal. Based on the geometries of the seating surface 40 and ball 38, there is a large radial distance between the inside diameter of the seating surface 40 and the outside diameter of the ball. Thus, the bearing area between the seating surface 40 and the ball 38 is small. As the ball 38 is loaded to successively higher loads, the ball 38 may be subjected to high compressive loads that exceed its material property limits, thereby causing the ball 38 to fail. Even if the ball 38 deforms, the ball 38 cannot deform enough to contact the tapered seating surface 40, and therefore the bearing surface 40 of the ball seat 36 for the ball 38 remains small. An increase in ambient temperature can also increase the likelihood of extruding the ball 38 through the seat due to decreased material properties. The mechanical properties of the ball 38 material may decrease, e.g., compressive stress limits and elasticity, which can lead to an increased likelihood of the ball cracking or extruding through the ball seat 36. Thus, in high temperature and high pressure environments, conventional isolation devices for frac plugs 30, i.e., balls 38 and ball seats 36 within the mandrel, may leak or fail.
When it is desired to remove one or more of these plugs from a wellbore, it is often simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the tool, or at least the outer components thereof, out of the well bore. In drilling, a drill bit or mill is used to cut and grind up the components of the plug to remove it from the wellbore.
Accordingly, there exists a need for an isolation device for a frac plug that effectively seals or isolates the zones above and below the plug in high temperature and high pressure environments.